Recently, for marking of letters and signs such as maker's name, product name, date of production, etc., on the surfaces of various commercial articles, for example, electronic parts such as IC's, resistors, condensers, inductors, etc., electrical parts such as relays, switches, connectors, printed circuit boards, etc., housings of electrical devices, automobile parts, machine parts, cables, sheets, packaging sheets, cards, various containers of foods, cosmetics, toiletaries and medicines, caps of containers, etc., the laser marking system is popularly employed for its various advantages such as non-contact quick marking and easiness of automation and process management.
In laser marking, laser beams are applied directly to the surface of an objective article made of a high-molecular organic material, metal or other substance to cause a change of state of the article surface or discoloration or decoloration of the colorant in the irradiated area, and marking is done by making use of such chemical changes. This marking method, however, has certain problems in practical applications. For example, in case of using a polyolefine resin as base material for marking, the laser beams may pass through the resin, failing to perform desired marking thereon, or even when the laser beams are absorbed to cause etching, it may happen that the resin be simply melted and no clear marking can be accomplished. Also, even when a colorant is used, vivid color development may fail to take place.
In order to solve these problems when a laser marking method is applied on polyolefin resin molded articles, many researches have been made on the color formers capable of developing vivid colors. For example, there have been proposed use of yellow iron oxide as color former in JP-A-60-155493, use of inorganic lead compounds in JP-A-61-69488 and JP-A-1-3062285, use of manganese violet and cobalt violet in JP-A-2-204888 and use of compounds of metals such as mercury, cobalt, copper, bismuth and nickel in U.S. Pat. No. 4,861,620. However, these compounds involve the serious problems in practical use, such as safety and influence on the environment, as they are basically composed of heavy metals. Also, in the case of heavy metals such as yellow iron oxides, since the compounds themselves are tinted, it is impossible to use resins colored in various hues and accordingly the scope of their use is limited. JP-A-60-110737 proposes use of pearlescent pigments as non-tinted compounds, but since these compounds have iridescent luster although not tinted, they may develop a foreign color tone when applied to an article for which no pearly luster is needed.
Even as a color former, there is no complete colorless one. Therefore, there are such problems that if the amount of color former increases, the products to be marked are influenced in hue and color tone, the viscosity of the resin composition increases and the resin composition is influenced in its moldability, the abrasion property increases and the molding machine and kneader are injured by the abrasion at the molding. Accordingly, even if the color former itself is colorless and its amount to be used is a very little, a marking composition to be colored in a clear dark brown to black by laser beam is required.